They were on an HH-60G Pave Hawk as part of a two-helicopter mission from RAF Lakenheath when it crashed.

Witness statements relating to the crash have been obtained by the BBC.

The investigation was initially hampered by the fact the helicopter was carrying munitions

Released by the United States Air Force in Europe under US freedom of information laws, they tell how the first of the two helicopters may have startled the pink-footed geese, which then went on to strike the one flying behind.

The Accident Investigation Branch previously confirmed a collision between geese and the helicopter had "rendered" both the pilot and co-pilot unconscious.

The helicopter was flying at 110ft (33m) above ground level at a speed of about 110 knots (126mph) just before the crash.

The impact was so fierce, the 1,867 pages of documents now released say, that parts of the birds hit were found entwined with some of the bolts from the downed twin-engine helicopter.

A second helicopter from RAF Lakenheath was sent to the marshes to try to assist

The co-pilot of the helicopter in front - whose name has been redacted - told a panel set up to investigate the crash: "The gunner said 'I've got a fireball at the six o'clock' or at the seven o'clock or something like that, and at that point we [tried] to initiate a radio contact with them and we failed to do it.

"Obviously, there was no answer.

"It did not look like what I thought a crashed helicopter would look like, you know, because there was obviously a lot of water there and stuff so [it] was a really small fire from my perspective."

Cley crash timeline

17:33 GMT: Take off

18:05: Birds startled on the ground and collide with the trailing HH-60G Pave Hawk

Three seconds later, the helicopter crashes

18:06 Lead helicopter realises it cannot see the downed helicopter and search begins

18:09 Lead helicopter lands at crash site

The lead pilot said: "Approaching the site I could start to see the black outline and a fire that was maybe, from that distance appeared to be 10 to 15 feet in height, but that's a pure guess.

"As we approached, the fire began to dissipate and got smaller and smaller as we came to it. By the time we landed it was almost out."

Asked about the birds, the gunner, said: "A lot of the time the problem is Chalk lead [the name of the lead helicopter] will actually fly over them on the ground and scare them up and then Chalk 2 [the second helicopter] will take the brunt of it."

Describing the lead-up to the crash, the gunner said: "I noticed they were gone.

"[I] saw the fire on the ground and we didn't see one when we flew over it.

"So I queried my flight engineer and they went over there and that's when the gut feeling that something was wrong hit me and I told the pilots, 'we need to spin around'."

The risk of bird strikes in the area was assessed as 'low'

A civilian witness on the ground, also interviewed by the panel, said: "It literally dropped, like incredibly suddenly.

"And I could see this orange glow, this orange illumination."

In his report on the crash, Brig Gen Jon Norman said: "A flock of geese took flight from Cley Marshes, likely startled by the noise of the approaching helicopters, and struck the [helicopter].

"At least three geese penetrated the windscreen."

The HH-60G Pave Hawk, a version of the US Army's Black Hawk helicopter, is used by the US Air Force for combat search and rescue, mainly to recover downed aircrew or other isolated personnel in war zones.

The helicopter, made by Sikorsky, has been used in numerous military missions in Iraq and Afghanistan, as well as civilian rescue operations after disasters including the 2004 Boxing Day tsunami in Sri Lanka and Hurricane Katrina in the US in 2005.

The Pave Hawk, which came into service in 1981, has a four-man crew and can carry up to 12 troops.